The invention relates generally to fingerprint readers and more particularly to systems for properly positioning a finger for imaging by a stationary camera or other imaging device.
Passwords and personal identification numbers (PINs) have been used to provide security for financial transactions or to limit data accessibility. As one example, a person may be required to provide a PIN prior to a bank withdrawal from an automatic teller machine (ATM). As another example, access to a computer may require?a user to input a particular password. A concern is that passwords and PINs may be freely used by others who legally or illegally gain access to the passwords or PINs. Another concern is that passwords and PINs are sometimes forgotten, leaving a person unable to engage in an authorized transaction or unable to gain authorized access to data.
Biometrics may be used as a supplement or a substitute for providing security by means of passwords and PINS. Biometric-based security relates to verifying a person""s identity by means of human features. The features may be facial features, such as the relative positions of eyes. Voiceprints may also be used to verify a person""s identity. Fingerprints provide perhaps the most reliable biometric identification. An advantage of biometric identification is that the physical characteristics of a person cannot be taken by another.
The implementation of fingerprint security involves a number of alternatives. The image information that is stored and used as a reference may be a direct mapping of all of the physical characteristics of a finger. On the other hand, additional security is provided by converting the direct image information into a map of minutiae points that represent irregularities within the fingerprint. In this implementation, only the minutiae points are stored, rather than the actual fingerprint. Authorization to engage in a transaction or to gain access to protected data then requires a person to hold a finger to a reader, which provides the image information used to create a second map of minutiae points that is compared to the reference map.
Another difference among fingerprint readers relates to whether the finger is flattened or the Henry roll technique is used. The Henry roll technique is similar to that used in law enforcement to obtain a wider area of information than is obtained when the finger is flattened.
Yet another difference among fingerprint readers relates to the equipment for acquiring the fingerprint information. An imaging device may be sufficiently large to allow both the imaging device and the finger to remain stationary. The imaging device may be a charge coupled device (CCD) or an array of tiny capacitive sensors which capture the fingerprint information by measuring the differences in electrical charges between fields and ridges of the skin. However, the size of the array must be relatively large if the array and the finger are to remain in fixed positions. This may be cost prohibitive in many applications of biometric security. Consequently, many fingerprint readers require relative movement between imaging device and the finger to be imaged.
U.S. Pat. No. 4,553,837 to Marcus describes a Henry roll fingerprint processing apparatus that requires relative movement between the finger and the imaging hardware. The apparatus includes optical scanning equipment, an array of photoelectric transducers, lenses, and electrical scanning equipment. A cylindrical-segment platen provides a transparent surface for supporting a finger. A motor rotates the light source, the array, the lenses and an element of the electrical scanning equipment. The axis of rotation is substantially the same as the axis of the finger-supporting surface of the platen. As the rotation occurs, the physical characteristics of the finger modulate the light beam to provide a reflected light beam having the desired fingerprint information.
It is important that the finger remain properly positioned relative to the imaging equipment. Typically, the desired orientation is one in which the finger is generally parallel to an imaging plane. The curvature of the finger-supporting surface of the Marcus apparatus is designed to promote proper placement of the finger to be imaged. Another apparatus of interest is described in U.S. Pat. No. 3,804,524 to Jocoy et al. The Jocoy et al. apparatus directs and positions a finger for fingerprint identification. The finger is placed so as to abut a forward stop member. As the finger pushes the abutted forward stop member, the finger slides along the surface of a flat window. An imaging device is on a side of the window opposite to the finger. Lateral movement of the finger is retarded by providing a guide member that is straddled by two fingers as the hand is moved along the surface of the apparatus. The desired flatness of the finger (i.e., a generally parallel relationship between the finger and the imaging plane) is achieved by providing a palm guide member that is allowed to move in the direction of hand movement. In operation, the palm of the person is placed in contact with the palm guide member such that two fingers straddle the lateral guide member. As the stop member is pushed from covering the window, the fingerprint information is acquired.
While the prior art techniques for promoting proper finger positioning improve the likelihood that sufficient fingerprint information will be acquired, further improvements are desired. For example, in the Jocoy et al. apparatus, the device may easily be operated with the hand being cupped. The cup-shaped hand will still contact the palm guide member and will allow the person""s finger to push the stop member from atop the window. However, the finger will be pointed toward the window as the finger travels over the imaging device. This adversely affects the likelihood that reliable fingerprint information will be acquired.
What is needed is a system that provides an increased reliability with respect to acquiring fingerprint information, with the increase being achieved as a consequence of promoting finger flatness during an imaging process.
A system for imaging a fingerprint includes providing a sliding finger guide that is restricted from following its established slide path, unless there is an applied force in the direction generally perpendicular to the slide path. The finger guide includes a finger-support region with an opening configured to expose a finger to an imaging device having a field of view that defines an interrogation region. In a preferred embodiment, the mechanism for restricting displacement of the finger guide is arranged to require pressure on opposite sides of the finger guide, with the opposite sides being aligned with the slide path. This alignment is intended to promote proper finger placement, since it requires finger contact at both the forward and rearward sides of the finger-supporting region.
In one embodiment, the finger guide is a carriage that is restricted from movement by a detent-and-notch arrangement in which at least one spring mechanism biases the finger guide into a locked position. In the locked position, detents rest in notches, so as to prevent sliding motion of the finger guide. However, by applying downward pressure onto the finger-supporting region of the guide, the detents are removed from the notches, allowing the guide to slide along a path that traverses the interrogation region of the imaging device. Typically, the imaging device is an optical arrangement, but this is not critical to the invention.
Preferably, the finger-support region has a central opening and has a contour designed to receive the finger to be imaged. For example, the contour may be the bottom portion of an elongated and truncated hemisphere. That is, the region may be cup-shaped, with an oval cross section that decreases in area as the cross section moves downwardly. Another preferred aspect of the finger guide is the use of raised bumps on the finger-support region in order to provide tactile feedback regarding finger positioning. For example, the-raised bumps may be on the forward and rearward sides of the finger-support region. Other tactile-feedback features may also be used.
In one embodiment, actuators extend from the surface of the finger-support region. Proper placement of a finger depresses the actuators and releases the finger guide for movement along its slide path. The actuators may be placed on the forward and rearward sides of the finger-support region, so that there is a strong likelihood that a finger is properly positioned when the guide is released from its locked position.
In another embodiment, proper finger positioning is promoted by using at least one roller that is locked unless certain conditions are met. For example, there may be a first roller on the forward side of an imaging window and a second roller on the rearward side of the imaging window, with the rollers being prevented from rotating unless pressure is applied to release members on the opposed lateral sides of the imaging window. The release members, rollers and imaging window should be sufficiently close to allow a single finger to contact all five components simultaneously. Consequently, when the finger is properly positioned, the two release members are depressed and the two rollers are allowed to rotate. In one application of this embodiment, the release members control the positions of projections which are aligned with teeth of gears at the ends of the rollers. Without pressure on the release members, the projections extend into the teeth and prevent gear rotation. However, when the release members are depressed, the projections are removed from contact with the gear teeth and the rollers rotate freely.
An advantage of the invention is that reliable fingerprint information is repeatedly acquired. The proper design of any one of the embodiments forces a user to properly position a finger to be imaged. The contour of the finger-support region and the raised bumps along that region provide tactile feedback for a user. Another advantage is that the locking mechanism reduces the likelihood of premature movement of the finger guide along its slide path. The user can adjust finger positioning prior to applying the pressure that releases the finger guide.